winsys/amdgpu: avoid potential segfault in amdgpu_bo_map()

cs can be NULL when it comes from r600_buffer_map_sync_with_rings()
to avoid doing the same checks. It was checked for write mappings
but not for read mappings.

Cc: "17.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit af303abcdbeac3b90fb760de19bed56cc40cfff4)
1 file changed